They are different than autobiographies in that they focus on a key theme in a person's life rather than a chronological retelling of the person's existence. Wikipedia clearly explains that autobiographies are of a life while memoirs are from a life.
Here are a few great memoir titles that might interest you:
The Last Lecture
Breaking Night
The Glass Castle
Night
The Diary of Anne Frank
Eat, Pray, Love
Into Thin Air: A Personal Account of the Mount Everest Disaster